Why would anyone think to pack a passport for a trip inside Washington state?

Yet that was the jolting advice my family received from U.S. Customs and Border Protection earlier this month as we returned from a weekend getaway to Friday Harbor on San Juan Island.

Our group — my husband and me; our 3-year-old daughter; and my mother, sister and nephew — arrived to catch the 1:40 p.m. Sunday ferry back to Anacortes. That happens to be the only Washington State Ferries run that originates in Canada; in this case, in Sidney, B.C., near Victoria. At the terminal, a flier left on the windshield of our minivan warned us that we'd be joining international passengers already on board and that all of us would be subject to a Customs search in Anacortes.

The flier advised us to take a later ferry if we didn't consent. But the next ferry to Anacortes wasn't for another three hours. So we decided to hop on. We figured that a "border crossing" for U.S. citizens returning from a domestic journey ought to be quick.

It wasn't.

Despite three vehicle exit lanes at the Anacortes dock, it took us an hour to crawl toward a Customs agent — the exact duration of our ferry ride. But what really grated was the grilling.

After my husband, David, finally pulled up, the Customs agent examined our driver's licenses. He asked David for his place of birth (Oklahoma). He asked for mine (Seoul, South Korea). He wanted to know if I was a naturalized citizen (yes), and since when (for 25 years). And why had I moved to the United States? (I was 11 then.)

Turning back to my husband, the agent asked David how long he'd known us. David must have looked startled, because the agent asked why he was perplexed.

Gathering his wits, David explained that we had traveled only from Anacortes to Friday Harbor and back, never leaving the United States. We hadn't expected an interrogation just to get back home to Kenmore.

The agent finally waved us off, but not before advising us to always carry copies of our passports to expedite our re-entry.

But why, when our trip stuck entirely to Washington?

According to Mike Milne, a U.S. Customs spokesman in Seattle, the federal government has legal authority to conduct inspections on all passengers who "co-mingle" on international vessels.

But according to the American Civil Liberties Union of Washington State, at least two courts here since 1990 have rejected the federal government's argument that Anacortes serves as a "functional border" for such ferry passengers.

Both rulings involved passengers who were caught with illegal drugs in their cars — cocaine and methamphetamine in one case and marijuana in the other — upon arrival in Anacortes. In both cases, the plaintiffs had told Customs agents they'd boarded the ferry in Friday Harbor.

Anyone crossing the international border to enter the U.S. can be searched without a warrant or probable cause. But the Washington Court of Appeals and the U.S. District Court in Western Washington both ruled in effect in the drug-seizure cases that Anacortes wasn't equivalent to a border.

A "functional border" requires actual border crossings, the courts wrote, which the government failed to prove in both cases because the majority of the passengers going through Customs in Anacortes were domestic travelers.

After the more-recent legal setback, in 2000, the federal government began issuing notification cards like the one we received to warn passengers that they are voluntarily consenting to an inspection.

Ironically, the federal government implicitly concedes the courts' logic by exempting those same ferry passengers from the new traveler-identification rules for international travel by land or sea (see Bring more ID to drive across borders on Travel's front page).

"Persons who travel on the 'domestic' leg of the 'foreign' ferry from British Columbia to Anacortes have not left the United States (for identification purposes), therefore, will not require the same documentation as those who do 'go foreign,' " Milne said.

Our encounter with the Customs agent in Anacortes struck me as even less justified when I learned that U.S. Customs agents had already pre-cleared passengers at the Sidney, B.C., ferry terminal. The inspection there focuses on intercepting would-be terrorists and illegal visitors, while the arrivals check at Anacortes mainly searches for banned cargo, Milne said.

"It's not duplicative, just layered," he said.

Steve Rodgers, director of marine operations for Washington State Ferries, said his office has been working with federal officials for a couple of years to screen passengers just once, in Sidney, B.C.

"It would expedite everything, and we wouldn't have the holdup in Anacortes," Rodgers said.

Rodgers said clearing Customs in Anacortes typically takes no more than 25 minutes, depending on the season. Our minivan was last in line, and it took us an hour.

The Sidney-Friday Harbor-Anacortes route runs in the fall and spring; it does not operate in the winter and in the summer skips the stop in Friday Harbor.

Rodgers could not say when the Anacortes Customs check would go away. Until then, heed this warning from Charlie Stemple, a program manager at U.S. Customs office in Seattle:

"Everyone coming into this country is an immigrant" until proven otherwise.